Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called plate precursor. In addition to the well-known photosensitive, so-called pre-sensitized plates, which are suitable for UV contact exposure through a film mask, also heat-sensitive printing plate precursors have become very popular in the late 1990s. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e. without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-) chemical process, such as ablation, polymerization, insolubilization by cross linking of a polymer, heat-induced solubilization or particle coagulation of a thermoplastic polymer latex.
The most popular thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating. The coating typically comprises an oleophilic binder, e.g. a phenolic resin, of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working) by the image-wise exposure. During processing, the solubility differential leads to the removal of the non-image (non-printing) areas of the coating, thereby revealing the hydrophilic support, while the image (printing) areas of the coating remain on the support. Typical examples of such plates are described in e.g. EP-A 625728, 823327, 825927, 864420, 894622 and 901902. Negative working embodiments of such thermal materials often require a pre-heat step between exposure and development as described in e.g. EP-625,728.
EP 1 985 445 discloses a lithographic printing plate precursor comprising on a support having a hydrophilic surface a coating comprising an IR absorbing agent and a contrast enhancing compound.
WO 2007/099047 discloses a heat-sensitive positive-working lithographic printing plate precursor comprising a support having a hydrophilic surface and a heat-sensitive coating comprising an IR absorbing agent, a phenolic resin and an alkaline soluble polymer comprising a specific monomeric unit including a sulfonamide group.
Unpublished patent application EP 8 105 202 discloses a heat-sensitive positive-working lithographic printing plate precursor comprising on a support having a hydrophilic surface, a heat-sensitive coating comprising an underlayer comprising a specific alkaline soluble polymer and an upperlayer comprising a phenolic resin.
Before, during and after the printing step, a lithographic printing plate is in general treated with various liquids such as for example ink and/or fountain solution or treating liquids for further improving the lithographic properties of the image and non-image areas. Ink and fountain solution for example, may attack the coating and may reduce the press life. It is of high importance that the coating is sufficiently resistant against the application of these variety of treating liquids or in other words, has a high chemical resistance. Furthermore, it is important that positive-working printing plate precursors based on a solubility difference exhibit a substantive differentiation between the development kinetics of exposed and non-exposed areas. In other words, the dissolution of the exposed areas in the developer should be completely finished before the unexposed areas also start dissolving in the developer. If this differentiation is not sufficiently pronounced, insufficient clean-out which may become apparent as a reduction of the sensitivity of the plate, toning (ink-acceptance in the non-image areas) and/or ink build-up on the blanket, may result. The low differentiation between the development kinetics of exposed and non-exposed areas may further lead to a loss of coating in the image areas, especially a loss of small image details or so-called high lights, a reduced press life and/or a reduced chemical resistance.